The spinodal instability is a prime signal for the first-order phase transition with negative speed of sound squared $cs^2$ in the Quantum-ChromoDynamics phase diagram relevant for the RHIC energy scan. In recent studies [1,2], one evolves planar unstable black branes dual to a plasma with a first order phase transition subject to the spinodal instability. Near a critical point the interface between cold and hot stable phases, given by its width and surface tension, features a wider phase separation and a smaller surface tension. Far away from a critical point the formation time of the spinodal instability is reduced. Across softer and harder phase transitions, I demonstrate that mergers of equilibrated peaks and unstable plateaux lead to the preferred final single phase separated solution.